Photoacoustic imaging method and photoacoustic imaging apparatus

ABSTRACT

A photoacoustic imaging method that enables photoacoustic images to be displayed at high speed is provided. The photoacoustic imaging method scans a subject with a light beam, detects acoustic waves generated within the subject due to the scanning of light to obtain acoustic wave detected signals, and generates volume data that represent three dimensional photoacoustic images of the subject based on the acoustic wave detected signals. Photoacoustic projection images projected in the direction of irradiation depth of the light are generated, prior to the volume data being generated and concurrently with the scanning of the light. The photoacoustic projection images are displayed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of Ser. No. 14/036,418filed Sep. 25, 2013, which is a bypass continuation of the PCTApplication No.: PCT/JP2012/001885 filed on Mar. 19, 2012, which claimsforeign priority from the Japanese Patent Application No. 2011-071382,filed in the Japanese Patent Office on Mar. 29, 2011 and the JapanesePatent Application No. 2012-051189, filed in the Japanese Patent Officeon Mar. 8, 2012, the disclosures of which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention is related to a photoacoustic imaging method, thatis, a method that irradiates light onto a subject such as living tissue,and images the subject based on acoustic waves which are generatedaccompanying the irradiation of light.

The present invention is also related to an apparatus that executes thephotoacoustic imaging method.

BACKGROUND ART

Conventionally, photoacoustic imaging apparatuses that image theinteriors of living organisms utilizing the photoacoustic effect areknown, as disclosed in U.S. Patent Application Publication No.20050004458 and X. Wang et al., “A High-Speed Photoacoustic TomographySystem based on a Commercial Ultrasound and a Custom Transducer Array”,Proc. of SPIE, Vol. 7564, pp. 75624-1-75624-9, 2010. Photoacousticimaging apparatuses irradiate pulsed light such as pulsed laser beaminto the living organisms. Biological tissue that absorbs the energy ofthe pulsed light generates acoustic waves by volume expansion thereofdue to heat. The acoustic waves are detected by an ultrasound probe orthe like, and visualizations of the interiors of the living organisms isenabled based on the electrical signals (photoacoustic signals) obtainedthereby.

Photoacoustic imaging methods construct images based only on acousticwaves that radiate from specific light absorbers. Therefore,photoacoustic imaging is favorably suited to image specific tissuewithin living organisms, such as blood vessels. Application ofphotoacoustic imaging to image and display blood vessels during surgeryhumans to enable confirmation of the positions of the blood vessels isbeing considered. In the case that photoacoustic imaging is applied tosuch a use, so called volume data that represent two dimensional regionsof subjects are generated based on photoacoustic signals that representtwo dimensional regions of the subjects, and tomographic images ofdesired cross sections are constructed based on the volume data, asdisclosed in U.S. Patent Application Publication No. 20100031662.

DISCLOSURE OF THE INVENTION

In the case that photoacoustic imaging is applied to confirm thepositions of blood vessels as described above, it is not necessary tochange a slice position a large number of times to observe a largenumber of tomographic images as in a case for medical diagnosis. It isdesired to display photoacoustic images expediently accompanyingmovement of a probe. However, in the conventional photoacoustic imagingmethod, volume data are generated, and then photoacoustic images aregenerated and displayed based on the volume data. Therefore, it had beendifficult to display photoacoustic images within short periods of time.

The present invention has been developed in view of the foregoingcircumstances. It is an object of the present invention to provide aphotoacoustic imaging method capable of displaying photoacoustic imagesat high speed.

Another object of the present invention is to provide a photoacousticimaging apparatus capable of executing such a photoacoustic imagingmethod.

A photoacoustic imaging method of the present invention comprises:

scanning a subject with light;

detecting acoustic waves generated within the subject by the scanning oflight to obtain acoustic wave detection signals; and

generating volume data that represent three dimensional acoustic imagesof the subject based on the acoustic wave detection signals;

photoacoustic projection images of the subject projected in theirradiation depth direction of the light being generated based on theacoustic detected signals prior to the volume data being generated andconcurrently with the scanning of the light; and

the photoacoustic projection images being displayed by a display means.

Note that the expression “concurrently with the scanning of the light”means that the timing at which the photoacoustic projection images aregenerated and the timing that the light is scanned overlap at leastpartially. By adopting this configuration, the photoacoustic projectionimages can be generated and displayed in a so called “real time” manneraccompanying the scanning of the light.

Note that in the photoacoustic imaging method of the present invention,it is desirable for:

the absolute values of the acoustic wave detection signals to beintegrated with respect to the irradiation depth direction of the light;and for

the photoacoustic projection images to be generated based on the valuesof the integrated acoustic wave detection signals.

In this case, it is desirable for a range in the direction ofirradiation depth within which the integration is performed to be ableto be set as desired.

In addition, in the photoacoustic imaging method of the presentinvention, it is desirable for:

photoacoustic tomographic images of planes that extend in theirradiation depth direction of the light to be generated based on theacoustic wave detection signals prior to the volume data being generatedand concurrently with the scanning of the light; and for

the photoacoustic tomographic images to be displayed by the displaymeans along with the photoacoustic projection images.

In addition, in the photoacoustic imaging method of the presentinvention, it is desirable for:

the subject to be scanned with acoustic waves concurrently with thescanning with light;

reflected acoustic wave detection signals to be obtained by detectingreflected acoustic waves which are reflected by the subject accompanyingthe scanning with acoustic waves;

reflected acoustic wave projection images which are projected in theirradiation depth direction of the light to be generated based on thereflected acoustic wave detection signals; and

the reflected acoustic wave projection images and the photoacousticprojection images to be displayed in an overlapping manner in a state inwhich common portions of the subject within the images overlap eachother.

In addition, in the photoacoustic imaging method of the presentinvention, it is desirable for:

the subject to be scanned with acoustic waves concurrently with thescanning with light;

reflected acoustic wave detection signals to be obtained by detectingreflected acoustic waves which are reflected by the subject accompanyingthe scanning with acoustic waves;

reflected acoustic wave tomographic images of planes that extend in theirradiation depth direction of the light to be generated based on thereflected acoustic wave detection signals; and

the reflected acoustic wave tomographic images and the photoacoustictomographic images to be displayed in an overlapping manner in a statein which common portions of the subject within the images overlap eachother.

In addition, in the photoacoustic imaging method of the presentinvention, it is desirable for:

the subject to be scanned with acoustic waves concurrently with thescanning with light;

reflected acoustic wave detection signals to be obtained by detectingreflected acoustic waves which are reflected by the subject accompanyingthe scanning with acoustic waves;

reflected acoustic wave tomographic images of planes that extend in theirradiation depth direction of the light to be generated based on thereflected acoustic wave detection signals; and

the reflected acoustic wave tomographic images and the photoacousticprojected images to be displayed by the display means.

In addition, in the photoacoustic imaging method of the presentinvention, it is desirable for:

images that represent blood vessels of living organisms to be generatedas the photoacoustic tomographic images.

A photoacoustic imaging apparatus of the present invention comprises:

light scanning means for scanning a subject with light;

acoustic wave detecting means for detecting acoustic waves generatedwithin the subject due to the scanning of light and obtaining acousticdetected signals;

means for generating volume data that represent three dimensionalphotoacoustic images of the subject based on the acoustic detectedsignals;

image constructing means for generating photoacoustic projection imagesof the subject projected in the irradiation depth direction of the lightbased on the acoustic detected signals prior to the volume data beinggenerated and concurrently with the scanning of the light; and

display means for displaying the photoacoustic projection images.

It is desirable for the photoacoustic imaging apparatus of the presentinvention to adopt a configuration, wherein:

the image constructing means is configured to be capable of generatingphotoacoustic tomographic images of the subject related to planes thatextend in the irradiation depth direction of the light based on theacoustic wave detection signals prior to the volume data being generatedand concurrently with the scanning of the light; and the photoacousticimaging apparatus further comprises:

image combining means for combining the photoacoustic tomographic imagesand the photoacoustic projection images such that the two types ofimages are displayed separately by the display means.

It is desirable for the photoacoustic imaging apparatus of the presentinvention to further comprise:

acoustic wave scanning means for scanning the subject with acousticwaves;

reflected acoustic wave detecting means for detecting acoustic wavesreflected by the subject due to the scanning of the acoustic waves andobtaining reflected acoustic detected signals; and

image combining means; and wherein:

the image constructing means is configured to be capable of generatingreflected acoustic projection images of the subject projected in theirradiation depth direction of the light based on the reflected acousticdetected signals prior to the volume data being generated andconcurrently with the scanning of the acoustic waves; and

the image combining means combines the reflected acoustic projectionimages and the photoacoustic projection images such that the two typesof images are displayed by the display means in an overlapping manner ina state in which common portions of the subject within the imagesoverlap each other.

It is desirable for the photoacoustic imaging apparatus of the presentinvention to further comprise:

acoustic wave scanning means for scanning the subject with acousticwaves;

reflected acoustic wave detecting means for detecting acoustic wavesreflected by the subject due to the scanning of the acoustic waves andobtaining reflected acoustic detected signals; and

image combining means; and wherein:

the image constructing means is configured to be capable of generatingreflected acoustic tomographic images of the subject related to planesthat extend in the irradiation depth direction of the light based on thereflected acoustic detected signals prior to the volume data beinggenerated and concurrently with the scanning of the acoustic waves; and

the image combining means combines the reflected acoustic tomographicimages and the photoacoustic projection images such that the two typesof images are displayed separately by the display means.

It is desirable for the photoacoustic imaging apparatus of the presentinvention to further comprise:

acoustic wave scanning means for scanning the subject with acousticwaves;

reflected acoustic wave detecting means for detecting acoustic wavesreflected by the subject due to the scanning of the acoustic waves andobtaining reflected acoustic detected signals; and

image combining means; and wherein:

the image constructing means is configured to be capable of generatingreflected acoustic tomographic images of the subject related to planesthat extend in the irradiation depth direction of the light based on thereflected acoustic detected signals prior to the volume data beinggenerated and concurrently with the scanning of the acoustic waves; and

the image combining means combines the reflected acoustic tomographicimages and the photoacoustic tomographic images such that the two typesof images are displayed by the display means in an overlapping manner ina state in which common portions of the subject within the imagesoverlap each other.

It is desirable for the photoacoustic imaging apparatus of the presentinvention to adopt a configuration, wherein:

the light scanning means is constituted by a holding portion that holdsa plurality of light irradiating sections that output the light towardthe subject and a plurality of detecting elements of the acoustic wavedetecting means arranged in a common single direction, and a movingmeans for moving the holding portion in a direction perpendicular to thesingle direction.

Alternatively, the photoacoustic imaging apparatus of the presentinvention may adopt a configuration, wherein:

the light scanning means is constituted by a plurality of lightirradiating sections which are arranged in a two dimensional matrix.

The photoacoustic imaging method of the present invention generatesphotoacoustic projection images projected in the irradiation depthdirection of the scanned light based on acoustic wave detection signalsconcurrently with the scanning of the light prior to generating volumedata, and displays the photoacoustic projection images on the displaymeans. Therefore, photoacoustic projection images can be generated anddisplayed more expediently compared to a case in which volume data aregenerated based on acoustic wave detection signals, and thenphotoacoustic images are generated based on the volume data.

The photoacoustic imaging method of the present invention may generatephotoacoustic tomographic images of planes that extend in theirradiation depth direction of the scanned light concurrently with thescanning of light prior to the volume data being generated, and displaysthe photoacoustic tomographic images along with the photoacousticprojection images on the display means. In this case, photoacoustictomographic images may also be generated and displayed expediently inaddition to the photoacoustic projection images.

The photoacoustic imaging method of the present invention may displaythe photoacoustic tomographic images and reflected acoustic wavetomographic images in an overlapped manner, or display the photoacousticprojection images and reflected acoustic wave projection images in anoverlapped manner. In this case, the positions of blood vessels and thelike can be more accurately discriminated, by referring to the reflectedacoustic wave images that represent living tissue.

The photoacoustic imaging method of the present invention may integratethe absolute values of photoacoustic signals in the irradiation depthdirection of light, and generate photoacoustic projection images basedon the integrated values of the photoacoustic signals. In addition, therange in the irradiation depth direction in which integration is to beperformed may be set as desired. In this case, the range in the depthdirection for which projection images are generated can be changed asdesired. By referring to photoacoustic projection images in such a case,whether a tissue system such as blood vessels which may be present alonga depth direction is at a position shallower than a predetermined depthor at a position deeper than the predetermined depth can be accuratelyunderstood.

The photoacoustic imaging apparatus of the present invention comprisesthe image constructing means that generates photoacoustic projectionimages, which are projected in the irradiation depth direction ofscanned light, of a subject based on acoustic wave detection signalsconcurrently with the scanning of the light and prior to volume databeing generated, and the display means that displays the photoacousticprojection images. Therefore, the photoacoustic imaging apparatus of thepresent invention is capable of executing the photoacoustic imagingmethod of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that illustrates the schematic configurationof a photoacoustic imaging apparatus that executes a photoacousticimaging method according to a first embodiment of the present invention.

FIG. 2 is a block diagram that illustrates a portion of theconfiguration illustrated in FIG. 1.

FIG. 3 is a flow chart that illustrates the flow of processes in thephotoacoustic imaging method.

FIG. 4 is a perspective view that illustrates a scanning mechanismemployed by the apparatus of FIG. 1.

FIG. 5 is a schematic diagram that illustrates an example of imagesdisplayed by the photoacoustic imaging method.

FIG. 6 is a diagram for explaining generation of a photoacousticprojection image.

FIG. 7 is a block diagram that illustrates the schematic configurationof a photoacoustic imaging apparatus that executes a photoacousticimaging method according to a second embodiment of the presentinvention.

FIG. 8 is a block diagram that illustrates a portion of theconfiguration illustrated in FIG. 7.

FIG. 9 is a block diagram that illustrates the schematic configurationof a photoacoustic imaging apparatus that executes a photoacousticimaging method according to a third embodiment of the present invention.

FIG. 10 is a block diagram that illustrates the schematic configurationof a photoacoustic imaging apparatus that executes a photoacousticimaging method according to a fourth embodiment of the presentinvention.

FIG. 11 is a block diagram that illustrates a portion of theconfiguration illustrated in FIG. 10.

FIG. 12 is a schematic diagram that illustrates an example of imagesdisplayed by the apparatus of FIG. 9.

FIG. 13 is a schematic diagram that illustrates an example of imagesdisplayed by the apparatus of FIG. 10.

FIG. 14 is a perspective view that illustrates a two dimensional probeemployed by a photoacoustic imaging method according to a fifthembodiment of the present invention.

FIG. 15 is a block diagram that illustrates the schematic configurationof a photoacoustic imaging apparatus that executes a photoacousticimaging method according to the fifth embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the attached drawings. FIG. 1 illustrates aphotoacoustic imaging apparatus 10 according to a first embodiment ofthe present invention. The photoacoustic imaging apparatus 10 includes:an ultrasound probe (probe) 11; an ultrasonic wave unit 12; and a laserlight source (laser unit) 13. The laser unit 13 generates a laser beamto be irradiated onto subject. The wavelength of the light beam to beirradiated onto subjects may be set as appropriate according to targetsof observation. The laser beam output by the laser unit 13 is guided tothe probe 11 by a light guiding means such as an optical fiber, thenirradiated onto subjects from the probe 11.

The probe 11 further outputs (transmits) ultrasonic waves to subjects asa type of acoustic wave, and detects (receives) reflected ultrasonicwaves reflected by the subjects. The probe 11 has a plurality ofultrasonic transducers which are arranged one dimensionally, forexample. The probe 11 also detects ultrasonic waves (acoustic waves)which are generated by targets of measurement within subjects absorbingthe laser beam output by the laser unit 13. The end portions of thelight guiding means, that is, the leading end portions of a plurality ofoptical fibers or the like, are arranged along the arrangement directionof the plurality of ultrasonic transducers, and the laser beam isirradiated toward the subjects therefrom.

Note that when photoacoustic images or ultrasonic images of subjects areobtained, the probe 11 is moved in a direction substantiallyperpendicular to the direction in which the ultrasonic transducers andthe end portions of the light guiding means extend, to thereby twodimensionally scan the subjects with the laser beam and the ultrasonicwaves. This scanning may be performed by an operator manually moving theprobe 11. Alternatively, a scanning mechanism such as that illustratedin FIG. 4 may be employed to realize more precise two dimensionalscanning. The scanning mechanism illustrated in FIG. 4 is constitutedby: a threaded engagement portion 80 which is coupled to the probe 11; aholding plate 81; a bore screw 82 which is rotatably held by the holdingplate 81 and is in threaded engagement with the threaded engagementportion 80; a guide rod 83 which is fixed to the holding plate 81 andpasses through an aperture of the threaded engagement portion 80; and amotor 84 for rotating the bore screw 82 in the clockwise andcounterclockwise directions. When the motor 84 of the scanning mechanismis driven, the threaded engagement portion 80 which is in threadedengagement with the rotating bore screw 82 advances and retreats, theprobe 11 moves in the direction of arrow W in FIG. 4, and the twodimensional scanning described above is realized.

Returning to FIG. 1, the ultrasonic wave unit 12 has a receiving circuit21, an A/D converting means 22, a reception memory 23, a data separatingmeans 24, photoacoustic image reconstructing means 25, a projectionimage generating section 60 that receives the output of thephotoacoustic image reconstructing means 25, an ultrasound imagereconstructing means 26, a tomographic image generating section 70 thatreceives the output of the ultrasound image reconstructing means 26 andthe output of photoacoustic image reconstructing means 25; and an imagecombining means 27 that receives the output of the tomographic imagegenerating section 70 and the output of the projection image generatingsection 60. The ultrasonic wave unit 12 further includes: a triggercontrol circuit 28, a transmission control circuit 30, and a controlmeans 31. The control means 31 controls each component of the ultrasonicwave unit 12.

The probe 11 detects the acoustic waves and outputs acoustic wavedetection signals. The probe 11 also detects the reflected ultrasonicwaves and outputs ultrasonic wave detection signals. The receivingcircuit 21 receives the acoustic wave detection signals and theultrasonic wave detection signals. The A/D converting means 22 is asampling means, and converts the acoustic wave detection signals and theultrasonic wave detection signals received by the receiving circuit 21into digital signals. The A/D converting means 22 samples each type ofsignal at a predetermined sampling period synchronized with an A/D clocksignal, for example.

The trigger control circuit 28 outputs a light trigger signal thatcommands light output to the laser unit 13. The laser unit 13 includes aflash lamp 32 which si a pumping light source for a Q switch pulse lasersuch as YAG or titanium sapphire, and a Q switch 33 that controls laseroscillation. When the trigger control circuit 28 outputs a flash lamptrigger signal, the laser unit 13 lights the flash lamp 32 and pumps theQ switch pulse laser. The trigger control circuit 28 outputs a Q switchtrigger signal after the flash lamp 32 sufficiently pumps the Q switchpulse laser, for example. The Q switch is turned ON when the Q switchtrigger signal is received, and causes a laser beam to be output fromthe laser unit 13. The amount of time required from the timing that theflash lamp 32 is lit to a point in time at which the Q switch laser issufficiently pumped can be estimated from the properties of the Q switchlaser.

Note that the Q switch 33 may be turned ON within the laser unit 13instead of the Q switch being controlled by the trigger control circuit28. In this case, a signal that indicates that a Q switch has beenturned ON may be transmitted to the ultrasonic wave unit 12 from thelaser unit 13.

In addition, the trigger control circuit 28 outputs an ultrasonic wavetrigger signal that commands ultrasonic wave transmission to thetransmission control circuit 30. The trigger control circuit 28 outputsthe light trigger signal first, and then outputs the ultrasonic wavetrigger signal thereafter. That is, the trigger control circuit 28outputs the ultrasonic wave trigger signal following output of the lighttrigger signal. Irradiation of a laser beam onto a subject and detectionof acoustic waves are performed by the light trigger signal beingoutput, and transmission of ultrasonic waves toward the subject anddetection of reflected ultrasonic waves are performed thereafter byoutput of the ultrasonic wave trigger signal.

The sampling control circuit 29 further outputs a sampling triggersignal that commands initiation of sampling to the A/D converting means22. The sampling control circuit 29 outputs a sampling trigger signal ata timing following output of a light trigger signal by the triggercontrol circuit 28 and prior to output of an ultrasonic wave triggersignal. The sampling control circuit 29 outputs a sampling triggersignal at a timing following output of the light trigger signal, andpreferably at a timing at which a laser beam is actually irradiated ontoa subject. For example, the sampling control circuit 29 outputs asampling trigger signal synchronized with the timing at which thetrigger control circuit 28 outputs a Q switch trigger signal. When thesampling trigger signal is received, the A/D converting means 22initiates sampling of ultrasonic waves (photoacoustic signals) detectedby the probe 11.

Following output of the light trigger signal, the trigger controlcircuit 28 outputs an ultrasonic wave trigger signal at a timing thatdetection of acoustic waves is completed. At this time, the A/Dconverting means 22 does not interrupt sampling of ultrasonic wavesignals, but continues to execute sampling. In other words, the triggercontrol circuit 28 outputs the ultrasonic wave trigger signal in a statein which the A/D converting means 22 is continuing sampling of theultrasonic wave signals. The target of detection of the probe 11 changesfrom acoustic waves to reflected ultrasonic waves, by the probe 11transmitting ultrasonic waves in response to the ultrasonic wave triggersignal. The A/D converting means 22 continuously samples thephotoacoustic waves and the reflected acoustic waves, by continuingsampling of detected ultrasonic wave signals.

The A/D converting means 22 stores both the sampled photoacousticsignals and the sampled reflected ultrasonic wave detection signals inthe common reception memory 23. The sampled data stored in the receptionmemory 23 are data of acoustic wave detection signals up to a certainpoint in time, and become data of reflected ultrasonic wave detectionsignals after the point in time. The data separating means 24 separatesthe acoustic wave detection signals and the ultrasonic wave signalsstored in the reception memory 23. The data separating means 24 providesthe separated acoustic wave detection signals to the photoacoustic imagereconstructing means 25, and provides the separated ultrasonic wavesignals to the ultrasound image reconstructing means 26.

The photoacoustic image reconstructing means 25 and the ultrasound imagereconstructing means 26 are capable of generating volume data thatrepresent three dimensional regions of subjects. However, in the methodof the present embodiment, projection images and tomographic images tobe described later are generated prior to the volume data beinggenerated. The functions regarding this point will be describedhereinafter.

The photoacoustic image reconstructing means 25 adds data from 64ultrasonic transducers of the probe 11 at delay times corresponding tothe positions of the ultrasonic transducers, to generate datacorresponding to a single line (delayed addition method), for example.Alternatively, the photoacoustic image reconstructing means 25 mayexecute image reconstruction by the CBP (Circular Back Projection)method. As further alternatives, the photoacoustic image reconstructingmeans 25 may execute image reconstruction by the Hough transform methodor Fourier transform method. The ultrasound image reconstructing means26 also generates data corresponding to each line of ultrasound images,which are tomographic images, from data generated based on theultrasonic wave detection signals.

FIG. 2 illustrates the detailed configuration of the projection imagegenerating section 60, into which reconstructed image data generated bythe photoacoustic image reconstructing means 25 are input, and thetomographic image generating section 70, into which reconstructed imagedata generated by the photoacoustic image reconstructing means 25 andreconstructed image data generated by the ultrasound imagereconstructing means 25 are input.

As illustrated in FIG. 2, the projection image generating section 60includes: a absolute value converting means 61, into which reconstructedimage data generated by the photoacoustic image reconstructing means 25are input; a depth direction data integrating means 62; a logarithmicconverting means 63; and a projection image constructing means 64, whichare connected in this order. The projection image generating section 60generates photoacoustic projection images projected in the irradiationdepth direction of the laser mean. Note that generation of thephotoacoustic projection images will be described in detail later.

Also as illustrated in FIG. 2, the tomographic image generating section70 includes: a detecting means 71, into which reconstructed image datagenerated by the photoacoustic image reconstructing means 25 are input;a logarithmic converting means 72; and a photoacoustic tomographic imageconstructing means 73, which are connected in this order. Thetomographic image generating section 70 further includes: a detectingmeans 75, into which reconstructed image data generated by theultrasound image reconstructing means 26 are input; a logarithmicconverting means 76; and a ultrasound tomographic image constructingmeans 77, which are connected in this order. The photoacoustictomographic image constructing means 73 and the ultrasound tomographicimage constructing means 77 are connected to an ultrasound/photoacousticimage combining means 74.

The detecting means 71 generates envelope curves of data that representeach line output by the photoacoustic image reconstructing means 25. Thelogarithmic converting means 72 logarithmically converts the envelopecurves to widen the dynamic ranges thereof. The photoacoustictomographic image constructing means 73 generates photoacoustictomographic images based on data that represent each line, on whichlogarithmic conversion has been administered. In greater detail, thephotoacoustic tomographic image constructing means 73 generatesphotoacoustic tomographic images by converting the positions of acousticwave detection signals (peak portions) along a temporal axis topositions in the depth direction of the photoacoustic tomographicimages, for example.

The detecting means 75, the logarithmic converting means 76, and theultrasound tomographic image constructing means 77 function in the samebasic manner as the detecting means 71, the logarithmic converting means72, and the photoacoustic tomographic image constructing means 73, andgenerate ultrasound tomographic images. The ultrasound tomographicimages, the photoacoustic tomographic images, and the photoacousticprojection images are generated concurrently with the scanning of thelaser beam.

The ultrasound/photoacoustic image combining means 74 receive data thatrepresent the photoacoustic tomographic images and data that representthe ultrasound tomographic images generated in the manner describedabove. The ultrasound/photoacoustic image combining means 74 combinesthe two types of images such that they will be displayed in anoverlapped state, in which common portions of the subject within theimages overlap each other. A combined tomographic image which isgenerated in this manner and a photoacoustic projection image generatedby the projection image generating section 60 are combined by the imagecombining means 27 of FIG. 1 such that they are displayed at differentpositions, and the images are ultimately displayed by the image displaymeans 14.

FIG. 3 is a flow chart that summarizes the flow of processes describedabove. First, a light trigger signal is output (A1), pumping of thelaser is initiated (A2), and a pulsed laser beam is output (A3). Next, asampling trigger signal is output (A4), sampling of acoustic wavedetection signals is initiated (A5), and the acoustic wave detectionsignals are stored in the reception memory (A6). Then, an ultrasonicwave trigger signal is output (A7), ultrasonic waves are transmitted(A8), reflected ultrasonic waves are received and ultrasonic wavedetection signals are sampled (A9), and the ultrasonic wave detectionsignals are stored in the reception memory (A10). Next, the acousticwave detection signals and the ultrasonic wave detection signals areseparated (A11), a photoacoustic projection image and a tomographicimage (an image in which a photoacoustic tomographic image and anultrasound tomographic image are overlapped) are generated (A12), thephotoacoustic projection image and the tomographic image are combinedsuch that they are displayed separately (A13), and the images aredisplayed by the image display means (A14).

FIG. 5 is a schematic diagram that illustrates an example of imageswhich are displayed by the image display means 14 by the processesdescribed above. As illustrated in the right side of FIG. 5, aphotoacoustic projection image represents a blood vessel distribution,for example. Here, the scanning of the laser beam is indicated by ahorizontal arrow. As the scanning processes, projection images of thescanned range are successively displayed. A photoacoustic tomographicimage related to a single cross section represented by the brokenhorizontal line is displayed at the left side of FIG. 5 as an image thatrepresents the position of a blood vessel when viewed in the crosssectional direction. In the present embodiment, the photoacoustictomographic image and the ultrasound tomographic image that representstissue systems of the subject (the portion indicated by hatching in FIG.5) are displayed in a positionally aligned and overlapped manner.Therefore, the positions of blood vessels within tissue systems can beclearly discriminated.

Here, generation of the photoacoustic projection images will bedescribed with reference to FIG. 2 and FIG. 6. As illustrated in theleft side of FIG. 6, if a single line L that extends in the laser beamirradiation depth direction within a tomographic image is considered,photoacoustic signals related to portions (blood vessels, for example)that expand and contract in volume due to light absorption will changefrom positive to negative (this applies to ultrasonic wave signals aswell). Therefore, the absolute value converting means 61 of FIG. 2obtains the absolute values of these signals, and the absolute values ofthe signals are integrated by the depth direction data integrating means62. The integrated value will correspond to an integrated value of lightabsorption of a portion along the line L. Accordingly, integrated valuesare obtained for the entire region which is scanned by light, and imagedby the projection image constructing means 64 illustrated in FIG. 2.Thereby, projection images that represent portions at which lightabsorption occurs can be obtained. Note that the logarithmic convertingmeans 63 illustrated in FIG. 2 is basically the same as the logarithmicconverting means 72.

As described above, the present embodiment generates photoacousticprojection images projected in the irradiation depth direction of thescanned light as well as photoacoustic tomographic images related to aplane that extends in the irradiation depth direction of the scannedlight based on acoustic wave detection signals concurrently with thescanning of the light prior to generating volume data, and displays theimages on the display means 14. Therefore, photoacoustic projectionimages and photoacoustic tomographic images can be generated anddisplayed more expediently compared to a case in which volume data aregenerated based on acoustic wave detection signals, and thenphotoacoustic images are generated based on the volume data.

Next, a photoacoustic imaging method according to a second embodiment ofthe present invention will be described. FIGS. 7 and 8 illustrate aphotoacoustic imaging apparatus 110 that executes the method of thesecond embodiment. FIG. 7 illustrates the basic configuration of thephotoacoustic imaging apparatus 110, and FIG. 8 illustrates a projectionimage generating section 60 and a tomographic image generating section170 of the photoacoustic imaging apparatus 110 in detail. Note that inFIGS. 7 and 8, constituent elements which are the same as thoseillustrated in FIGS. 1 and 2 are denoted by the same reference numerals,and detailed descriptions thereof will be omitted insofar as they arenot particularly necessary (this applies to all following embodiments).

The method of the present embodiment does not generate ultrasoundimages. Comparing the photoacoustic imaging apparatus 110 with thephotoacoustic imaging apparatus 10 of FIG. 1, the data separating means24 and the ultrasound image reconstructing means 26 are omitted, and thetomographic image generating section 170 which is illustrated in detailin FIG. 8 is employed instead of the tomographic image generatingsection 70. Note that the projection image generating section 60 is thesame as that employed in the photoacoustic imaging apparatus 10 of FIG.1.

The tomographic image generating section 170 is basically constituted bythe detecting means 71, the logarithmic converting means 72, and thephotoacoustic tomographic image constructing means 73 illustrated inFIG. 2, and only generates photoacoustic tomographic images. That is,ultrasound tomographic images are not generated, and thereforephotoacoustic tomographic images are not displayed in an overlappedmanner with ultrasound tomographic images. Photoacoustic tomographicimages generated by the tomographic image generating section 170 aredisplayed along with photoacoustic projection images generated by theprojection image generating section 60 by the image display means 14.

In the present embodiment as well, photoacoustic projection imagesprojected in the irradiation depth direction of the scanned light aswell as photoacoustic tomographic images related to a plane that extendsin the irradiation depth direction of the scanned light are generatedbased on acoustic wave detection signals concurrently with the scanningof the light prior to generating volume data, and the photoacousticimages are displayed on the display means 14. Therefore, photoacousticprojection images and photoacoustic tomographic images can be generatedand displayed more expediently compared to a case in which volume dataare generated based on acoustic wave detection signals, and thenphotoacoustic images are generated based on the volume data.

The photoacoustic imaging method of the present invention is not limitedto the display formats for projection images and tomographic imagesdescribed above, and the images may be displayed in other formats. Table1 below illustrates examples of alternate display formats. Note that inTable 1, “Photoacoustic+Ultrasound” refers to overlapped display of thetwo types of images.

TABLE 1 Display Format Tomographic Images Projection Images 1Photoacoustic Photoacoustic 2 Ultrasound Photoacoustic 3 Photoacoustic +Photoacoustic Ultrasound 4 Photoacoustic + Photoacoustic + UltrasoundUltrasound

The first embodiment which was described previously employs DisplayFormat 3 of Table 1, and the second embodiment employs Display Format 1.

Next, a photoacoustic imaging method according to a third embodiment ofthe present invention will be described. FIG. 9 illustrates the basicconfiguration of a photoacoustic imaging apparatus 210 that executes themethod of the third embodiment. Comparing the photoacoustic imagingapparatus 210 with the photoacoustic imaging apparatus 10 of FIG. 1, adisplay image generating means 227 is provided instead of the imagecombining means 27, and an integrating depth setting means 220 isfurther provided.

The integrating depth setting means 220 sets the depth from the surfaceof a subject, for example, to which the depth direction data integratingmeans 62 of the projection image generating section 60 (refer to FIG. 2)performs integrating processes. For example, the integrating depthsetting means 220 is constituted by a keyboard for inputting depths asnumerical values, a mouse that moves a cursor displayed by the imagedisplay means 14 that indicates a depth position, or the like. Dataregarding the integrating depth set by the integrating depth settingmeans 220 are input to the control means 31. The control means 31controls the integrating process performed by the depth direction dataintegrating means 62 such that data are integrated to a position at adepth indicated by the input data.

The display image generating means 227 causes photoacoustic projectionimages generated by the projection image generating section 60 for theset integrating depth and ultrasound tomographic images generated by thetomographic image generating section 70 by the image display means 14 inso called “real time” concurrently with scanning by the probe 11, forexample. Note that as described previously, “concurrently” means thatthe timing at which the photoacoustic projection images are displayedand the timing that the light is scanned overlap at least partially.

FIG. 12 illustrates an example of an ultrasound tomographic image and aphotoacoustic projection image which are displayed in the mannerdescribed above. Note that in this example, the aforementioned cursor CSis displayed within the ultrasound tomographic image illustrated at theleft side of FIG. 12.

In this example, if the integrating depth is not set, that is, if theintegrating depth is not limited, a blood vessel distribution thatextends from the upper side of the drawing sheet will be displayed inregion RG of the photoacoustic projection image at the right side ofFIG. 12. In contrast, by setting the integrating depth, blood vesselswhich are present at positions deeper than the set depth will not bedisplayed, as illustrated in FIG. 12. Accordingly, if the depth is setto the maximum depth that could be cut by a scalpel during surgery, allblood vessels displayed in the photoacoustic projection image can berecognized as those that may possibly be cut by a scalpel. In such acase, blood vessels being cut by scalpels can be positively prevented byavoiding the blood vessels displayed in the photoacoustic projectionimage during surgery.

In contrast, in the case that the integrating depth is not set, allblood vessels, including those present at positions too deep to be cutby a scalpel, are displayed in photoacoustic projection images.Therefore, locations that need not be avoided may also be discriminatedas locations that should not be cut by a scalpel. Further, cases inwhich surgeons mistakenly think that they have cut blood vesselsalthough in actuality, the blood vessels are not cut because they are atdeep positions are also entirely possible. Therefore, a problem arises,that regions to be avoided when cutting with scalpels become ambiguous.This problem can be prevented from occurring by setting the integratingdepth as described above.

Next, a photoacoustic imaging method according to a fourth embodiment ofthe present invention will be described. FIG. 10 illustrates the basicconfiguration of a photoacoustic imaging apparatus 310 that executes themethod of the fourth embodiment. Comparing the photoacoustic imagingapparatus 310 with the photoacoustic imaging apparatus 210 of FIG. 9,the photoacoustic imaging apparatus 310 is different in that aprojection image generating section 360, the detailed configuration ofwhich is illustrated in FIG. 11, is provided instead of the projectionimage generating section 60.

The projection image generating section 360 illustrated in FIG. 11differs from the projection image generating section 60 of FIG. 2 inthat the projection image generating section 360 further includes: ashallow portion data selecting means 300 provided prior to theprojection image constructing means 64, a shallow portion projectionimage constructing means 301 that receives input from the shallowportion data selecting means 300, and a whole projection image/shallowportion projection image combining means 302 that receives input fromthe projection image constructing means 64 and the shallow portionprojection image constructing means 301.

The depth direction data integrating means 62 of the projection imagegenerating section 360 performs integrating processes without setting anintegrating depth, that is, without limiting the integrating depth.Accordingly, the projection image constructing means constructsphotoacoustic projection images (whole projection images) in which theintegrating depth is not limited, in the same manner as thephotoacoustic imaging apparatus 10 of the first embodiment illustratedin FIG. 1.

Data output from the logarithmic converting means 63 is also input tothe shallow portion data selecting means 300. The shallow portion dataselecting means 300 selects and extracts data up to a predetermineddepth. The selected and extracted data are output to the shallow portionprojection image constructing means 301. Note that the depth to whichdata are selected is set by the integrating depth setting means 220 ofFIG. 10. The shallow portion projection image constructing means 301employs only data related to the selected shallow portions to constructphotoacoustic projection images (shallow portion projection images). Theshallow portion projection images which are constructed in this mannerare similar to projection images which are constructed after the depthdirection data integrating means 62 performing integrating processeswith a limited integrating depth.

The whole projection images constructed by the projection imagesconstructing means 64 and the shallow portion projection imagesconstructed by the shallow portion projection image constructing means301 are combined by the whole projection image/shallow portionprojection image combining means 302, and the combined projection imagesare displayed by the image display means 14 of FIG. 10. FIG. 13illustrates an example of a combined and displayed photoacousticprojection image along with an ultrasound tomographic image. Here, aprojection image (shallow portion projection image) similar to that inthe example illustrated in FIG. 12 is displayed, and a whole projectionimage VG is displayed in a region RG, at which nothing is displayed inthe example of FIG. 12. The whole projection image VG is displayed at alower density or in a display color different from that of the shallowportion projection image so as to be distinguishable. The imagedisplayed by the image display means 14 of the present embodimentenables discrimination of what is present up to a predetermined depth asin the display example of the third embodiment illustrated in FIG. 12,and also enables discrimination of what is present at deeper regions.

Next, a photoacoustic imaging apparatus according to a fifth embodimentof the present invention will be described. FIG. 14 illustrates a twodimensional ultrasound probe (probe) 411 which is employed in the methodof the fifth embodiment. FIG. 15 illustrates the basic configuration ofa photoacoustic imaging apparatus 410 that executes the method.Comparing the photoacoustic imaging apparatus 410 with the photoacousticimaging apparatus 210 of FIG. 9, the photoacoustic imaging apparatus 410is different in that a the two dimensional probe 411 is employed insteadof the probe 11, and the probe scanning mechanism 15 is omitted.

As illustrated in FIG. 14, the two dimensional probe 411 is of astructure in which a plurality of light output portions 400 such as theaforementioned light output ends of optical fibers are arranged in a twodimensional matrix. Note that ultrasonic transducers for detectingacoustic waves generated due to light irradiation are omitted from FIG.14, but are provided paired with each light output end, or provided tosurround the plurality of light output portions 400 which are arrangedin a two dimensional matrix. The two dimensional probe 411 is capable ofirradiating pulsed laser beams corresponding to each cross section ontoa subject, as schematically illustrated by the broken line in FIG. 14.The positions onto which light is irradiated can be sequentially changedin a direction perpendicular to an irradiating surface.

That is, the two dimensional probe 411 enables subjects to be scannedwith pulsed laser beams without employing a scanning mechanism such asthat illustrated in FIG. 4. Photoacoustic projection images aregenerated by the projection image generating section 60 based on datacorresponding to each cross section obtained by scanning the subject inthe same manner as in the previously described embodiments. Thegenerated projection images are displayed by the image display means 14in real time.

Note that light may be irradiated from all of the light output portions400 which are arranged in a two dimensional simultaneously, and data maybe obtained for each cross section instead of irradiating lightcorresponding to each cross section of a subject.

Preferred embodiments of the present invention have been describedabove. However, the photoacoustic imaging apparatus and thephotoacoustic imaging method are not limited to the above embodiments.Various changes and modifications to the configurations of the aboveembodiments are included in the scope of the present invention.

What is claimed is:
 1. A photoacoustic imaging method, comprising:scanning a subject with light; detecting acoustic waves generated withinthe subject by the scanning of light in order to obtain acoustic wavedetection signals; generating a two-dimensional photoacoustic projectionimage of the subject line by line, each line of the two-dimensionalphotoacoustic projection image being projected in an irradiation depthdirection of the light, wherein the generating of each line is based onthe acoustic detected signals prior to volume data being generated andconcurrently with the scanning of the light; generating the volume datathat represent a three dimensional acoustic image of the subject basedon the acoustic wave detection signals; and displaying the photoacousticprojection image by a display section line by line.
 2. A photoacousticimaging method as defined in claim 1, further comprising: integratingabsolute values of the acoustic wave detection signals with respect tothe irradiation depth direction of the light; and wherein the each lineof the photoacoustic projection image is generated based on theintegrated values of the absolute values of acoustic wave detectionsignals.
 3. A photoacoustic imaging method as defined in claim 2,further comprising: setting a range in the direction of irradiationdepth within which the integration is performed as desired.
 4. Aphotoacoustic imaging method as defined in claim 1, further comprising:generating photoacoustic tomographic images of planes that extend in theirradiation depth direction of the light based on the acoustic wavedetection signals prior to the volume data being generated andconcurrently with the scanning of the light; and displaying thephotoacoustic tomographic images by the display section along with thephotoacoustic projection image.
 5. A photoacoustic imaging method asdefined in claim 4, further comprising: scanning the subject withacoustic waves concurrently with the scanning with light in order toobtain reflected acoustic wave detection signals; obtaining reflectedacoustic wave detection signals by detecting reflected acoustic waveswhich are reflected by the subject accompanying the scanning withacoustic waves; generating reflected acoustic wave tomographic images ofplanes that extend in the irradiation depth direction of the light basedon the reflected acoustic wave detection signals; and displaying thereflected acoustic wave tomographic images and the photoacoustictomographic images in an overlapping manner in a state in which commonportions of the subject within the images overlap each other.
 6. Aphotoacoustic imaging method as defined in claim 4, further comprising:generating images that represent blood vessels of living organisms asthe photoacoustic tomographic images.
 7. A photoacoustic imaging methodas defined in claim 1, further comprising: scanning the subject withacoustic waves concurrently with the scanning with light; obtainingreflected acoustic wave detection signals by detecting reflectedacoustic waves which are reflected by the subject accompanying thescanning with acoustic waves; generating a reflected acoustic waveprojection image of the subject line by line, each line of the reflectedacoustic wave projection image being projected in the irradiation depthdirection of the light based on the reflected acoustic wave detectionsignals; and displaying the reflected acoustic wave projection image andthe photoacoustic projection image in an overlapping manner line by linein a state in which common portions of the subject within the imagesoverlap each other.
 8. A photoacoustic imaging method as defined inclaim 1, further comprising: scanning the subject with acoustic wavesconcurrently with the scanning with light in order to obtain reflectedacoustic wave detection signals; obtaining reflected acoustic wavedetection signals by detecting reflected acoustic waves which arereflected by the subject accompanying the scanning with acoustic waves;generating reflected acoustic wave tomographic images of planes thatextend in the irradiation depth direction of the light based on thereflected acoustic wave detection signals; and displaying the reflectedacoustic wave tomographic images and the photoacoustic projected imageby the display section.
 9. The photoacoustic imaging method of claim 1,wherein: the subject is continuously scanned with the light andconcurrently the each line of the two dimensional photoacousticprojection image is generated such that while a second cross section ofthe subject is being scanned, a line of the two dimensionalphotoacoustic projection image for a first cross section of the subjectis generated.